1. Field of the Invention
The invention relates to design improvements in electrostatic chucks. More particularly, the invention relates to an RF electrode contact assembly for a detachable electrostatic chuck.
2. Description of the Background Art
In semiconductor wafer processing equipment, substrate supports are used for retaining wafers during processing. The wafer rests on a susceptor such as an electrostatic chuck. Electrostatic chucks secure a workpiece by creating an electrostatic attractive force between the workpiece and the chuck. A voltage, applied to one or more insulated electrodes in the chuck, induces opposite polarity charges in the workpiece and electrodes respectively. The opposite charges pull the workpiece against the chuck, thereby retaining the workpiece.
The susceptor is typically mounted to a pedestal in a semiconductor processing chamber. These susceptors find use in etching, chemical vapor deposition (CVD), and physical vapor deposition (PVD) applications. In many applications, the susceptor is heated to high temperatures, typically as high as 175.degree. C. and sometimes as high as 500.degree. C. In many applications, the susceptor contains a cathode electrode that can be biased with direct current (DC) or radio frequency (RF) voltage. Bias voltage is typically supplied by an external power supply through a cable and appropriate feed throughs.
An improvement in the design of susceptors is a detachable electrostatic chuck such as that shown and described in commonly assigned U.S. Application Ser. No. 09/071,784 filed May 1, 1998. Such a chuck is secured to a pedestal but easily removable to facilitate repair and replacement of the chuck. When a bias must be applied to a detachable chuck, the chuck design must incorporate a releasable connection that makes electrical contact between the cathode and the cable. To ensure good electrical coupling, the releasable connection should make intimate contact at multiple points. Furthermore, the connector should maintain good electrical contact over a wide range of temperatures. Prior art connections have typically made use of "banana" plugs that have a male connector with one or more resilient contact portions that fits into a cylindrical female connector. The resilient contacts make good electrical connection at multiple points. Unfortunately, the male and female connectors must be precisely manufactured and aligned during assembly to ensure consistent connection and disconnection. Repeated connection and disconnection can bend the male connectors out of alignment making connection difficult. Furthermore, the male connectors can break after repeated bending thereby rendering the chuck useless.
One solution to this problem, is to use spring loaded connectors. Typically, a spring loaded connection consists of a coil spring secured to a fixed connection on the pedestal. A conductive button is attached to the free end of the spring. When the chuck is attached to the pedestal, the spring urges the button against a contact on the chuck thereby making the desired electrical connection. For example, U.S. Application Ser. No. 09/071,784 describes a large spring loaded plate that contacts the bottom of the electrostatic chuck to achieve an RF connection. However, such a plate (used for cooling) is not always necessary and may render construction of the chuck and pedestal unduly complex. This type of connector does not require the precise alignment of a banana plug connection. Although this type of connector works well for a DC bias, the impedance of the coil spring produces a sizable voltage drop when a high frequency (e.g. 13.56 MHz) RF voltage is applied.
Therefore, a need exists in the art for a spring loaded releasable RF connection for a substrate support that does not require precise alignment or unduly bulky components.